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Chapter 23
Populations or Species Evolution
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Questions?
What is the smallest unit of natural selection?
What is the smallest unit of evolution?
What is the difference between species and populations?
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Mas Evolution Questions
What is the basis of evolution?
What are the likely sources of population variation?
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Part 1
Sources of Variation
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Mechanisms of Evolutionary Change
Mutations provide the foundation for evolution Certain mutations will be selected for
No mutations is part of Hardy-Weinberg Equilibrium (No Evolution Equilibrium)
Only cell line mutations are able to be passed to offspring
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Mutations
Mutation is any change in an organism’s DNA
2 Types of Genetic Mutations Point Mutations
Changes in 1 base of a gene Base-Pair Substitutions
Result in 1 of 3 types of Mutations Nonsense Missense Silent
Insertion & Deletions May result in a Frameshift Mutation
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Chromosomal Mutations
Mutation that affects multiple genes (multiple loci) at once
There are 4 types:1. Duplication2. Deletion3. Reciprocal Translocation4. Inversion
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Sexual Recombination
Majority of genetic variation
New combinations of already existing alleles in the genome
Every generation experiences sexual recombination
3 Mechanisms of Sexual Recombination1. Crossing over (When?)2. Independent Assortment of Chromosomes3. Fertilization
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Part 2
Mechanisms of Evolution
*** Evolution is a change in allelic frequencies in a population or gene pool.
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Mechanisms of Evolution?
Natural Selection Most important mechanism of change in allelic
frequencies in a population (Evolution)
Genetic Drift Random, nonadaptive change
Gene Flow Immigration or emigration that changes allelic frequencies
Nonrandom Mating Selective mating, Sexual Selection, Artificial selection
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Genetic Drift
If I flip a coin 1,000 times, how many times will it come up “heads”? What will be the side frequencies?
If I flip a coin 10 times, how many times will it come up “heads”? What will be the side frequencies?
So the smaller the population the greater the deviation from the expected results is possible
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Genetic Drift (Page 2)
Random Changes in allelic frequencies large changes in allelic frequencies from one generation to the next
SMALL populations When a question concerns a shrinking of a populations = Genetic drift
Unpredictable changes in allelic frequencies
Random & Nonadaptive change So it causes evolution, but does not make the population better
adapted to their environment
2 types: Bottleneck effect Founder’s effect
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Bottleneck Effect
Sudden environmental change drastically reduces the population size.
The few survivors have passed through a restrictive “bottleneck”
The few survivors have a gene pool that is different than the original population’s.
Population bottlenecks always reduce genetic variation
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Bottleneck Effect (Page 2)
In 1890s, humans reduced elephant seal population to 20
In the next 100 years, they were protected 20 30,000 elephant seals
Researchers investigated 24 loci and found NO variation Comparable species populations display significant
variation at these alleles
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Founder Effect
A small part of the population is separated from the rest of the population, and establishes a new population
The new population has a different gene pool and allelic frequencies than the original population
Colonization of a new land
Founders pass through an “isolation bottleneck”
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Founder’s Effect (Page 2)
Associated with questions on Island Populations + Reduced variation
In 1814, 15 British colonists founded settlement on Tristan de Cunha (Islands between Africa & South America) Retinitis Pigmentosa is ten times higher in this island
population than in the original population
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Gene Flow
Movement of fertile individuals or gametes into or out of the population
May gain or loose alleles
Tends to reduce differences between populations
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Nonrandom Mating
Example: In plants, insect pollination of particular types of plants in the population Speciation
Self-fertilization Remember Mendel?
Sexual Selection Changes allelic frequencies & results in adaptations, just
not to the environment
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Natural Selection
Only mechanism that causes adaptive evolution
Increases the frequency of certain genotypes
But natural selection acts on or selects for particular phenotypes Acts indirectly on genotypes
Fitness – reproductive contribution of a phenotype to subsequent generations An individual’s fitness is determined by the probability of
survival of offspring times the number of offspring produced over their lifetime
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The effect of Natural Selection
Most characters are influenced by alleles at multiple locations, hence they are considered quantitative traits Body size
Natural selection can affect quantitative traits in 3 main ways: Directional selection Stabilizing selection Disruptive selection
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Individuals toward one extreme of a phenotypic range are favored
Common response to an environmental change
Avg. size of black bears increased each glacial period
Larger bears = better insulation during cold times
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Extreme phenotypes are selected against
Reduces variation & perpetuates status quo
Birth weights of human babies
Large birth weight or small birth weight have higher infant mortalities
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Individuals with extreme phenotypes are selected for.
Small beaked birds feed successfully on soft seeds, but large billed birds feed on rigid seeds, so intermediate beak lengths are selected against.
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Maintaining Genetic Variation
Selection & genetic drift decrease variation, so variation should be decreased over time?
Diploidy Recessive alleles retain variation, but are hidden esp. in
heterozygotes Natural Selection selects phenotypes, not genotypes
Neutral alleles Some alleles have no net effect on fitness, but they
introduce additional variance into a population
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Maintaining Genetic Variation (P2)
Sexual Recombination/Reproduction Not all species reproduce sexually Crossing over, Independent Assortment, &
Recombination during fertilization
Balancing Selection Natural selection may maintain stable frequencies of 2
or more phenotypic forms in a population This is called balanced polymorphism
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Heterozygote Advantage
Sometimes heterozygous have higher fitness than either homozygote
Natural selection will maintain both alleles
Consider the case of sickle-cell anemia + Malaria
Malaria (Plasmodium vivax) successfully infects normally shaped RBCs only!!
Premature rupturing of cell membrane Abnormally formed hemoglobin is indigestible to the
plasmodium parasite
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Heterozygote Advantage (P2)
Homozygote Dominant (AA)
Heterozygote (Aa)
Homozygote Recessive (aa)
Sickle-cell RBCs
0% 50%* 100%
Normal RBCs 100% 50%* 0%
Sickle-cell Symptoms
Never Certain Circumstances
Yes
RBCs Susceptible to Malarial infection
100% 50% 0%
* I made up this number, it is actually an intermediate value and varies widely in the population, but 50% is such a nice number.
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Heterozygote Advantage (P3)
Heterozygotes are relatively unaffected by sickle-cell anemia, but also have a percentage of cells that are protected from malarial infection As long as heterozygotes stay near sea level, no sickle-
cell symptoms, and reduced severity of malarial symptoms
Q.E.D. Heterozygote advantage
So the recessive allele’s (sickle-cell anemia) harm is balanced by its benefit in reducing or preventing malarial infection
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Sickle-Cell Disease
Malarial Disease
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Why hasn’t evolution produced perfect organisms yet ?
Evolution is limited by historical constraints New structures are not completely new, rather they are
modifications of existing structures Similar to modifying an existing building or just tearing it
down and building another one
Adaptations are often compromises Animals typically perform a myriad of functions, so
adaptations are rarely designed perfectly for many functions Knees, make sense for quadrupeds, but suck for aging human
bipeds & UM running backs (Gore, McGahee, Cooper, etc.)
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Evolution ain’t perfecto
Chance and Natural Selection Interact Gene Flow or Gene Drift are created by random selective
events Alleles present in the new population are not necessarily
best suited to the environment
Selection can edit only the existing variation Natural selection favors only the fittest phenotypes
currently in the population These traits may not be ideal